A power supply system (e.g., an adaptor) is used to convert an alternating current (ac) voltage from the utility company into a direct current (dc) voltage suitable for electronic devices. The power supply system usually includes an ac/dc stage (e.g., a rectifier) and an isolated dc/dc stage (e.g., an isolated dc/dc converter). The ac/dc stage converts the power from the ac utility line and establishes a dc bus for the isolated dc/dc stage. The ac/dc stage may comprise a variety of electromagnetic interference (EMI) filters and a bridge rectifier formed by four diodes. The EMI filters are employed to attenuate both differential mode noise and common mode noise. The bridge rectifier converts the ac voltage into a full-wave rectified dc voltage. Such a full-wave rectified dc voltage provides a steady dc input voltage for the isolated dc/dc stage through a plurality of smoothing capacitors coupled to the output of the bridge rectifier.
The isolated dc/dc stage converts the voltage of the dc bus to a voltage suitable to electronics loads such as tablets, printers, mobile phones, personal computers, any combinations thereof and the like. The isolated dc/dc stage can be implemented by using different power topologies, such as flyback converters, forward converters, half bridge converters, full bridge converters and the like.
In some applications (e.g., an adaptor for powering a personal computer), a flyback converter is employed to regulate the output voltage. The flyback converter includes a transformer, which provides galvanic isolation for satisfying various safety requirements. The flyback converter may comprise three controllers, namely a primary side controller placed at the primary side for driving a main switch of the flyback converter, a synchronous rectifier controller placed at the secondary side for controlling the on and off of the synchronous switch to reduce secondary side conduction losses, and a secondary side controller placed at the secondary side for sensing the output voltage and communicating with the primary side controller for achieve various system functions such as closed-loop regulation, universal serial bus (USB) power delivery protocols and the like.
All three controllers above may have their individual internal linear or low drop out (LDO) regulators to maintain a regulated bias voltage. In order to meet USB 3.0 type C Power Delivery (PD) specification, the output voltage of the flyback converter is in a wide range from about 3 V to about 20 V to. Such a wide output voltage range may cause extra power losses at the LDO regulators.